Important heterosis on hybridization
Abstract
Mahdi Tajalifar
An understanding of heterosis in genetic terms had to await the rediscovery of Mendel’s laws in 1900. It was immediately apparent that hybrids are more heterozygous than their parents. A decrease in the number of heterozygote’s implied an increase in the number of homozygote’s. This immediately gave rise to two explanations. The `dominance` hypothesis notes that most recessive mutants are deleterious, so inbred lines are weakened by having an increase in the number of homozygous recessive genes. Hybrids, in contrast, are stronger because the recessives from each parent are usually concealed by dominants from the other. The over dominance hypothesis assumes that there are some loci at which the heterozygote is superior to either homozygote. Although the two ideas are not mutually exclusive, the dominance hypothesis is now generally favored. This explanation also applies to variety and species hybrids, because the hybrids are always more heterozygous than their parents, the more so as the parents diverge. The contrast is greatest, however, when the parents are highly homozygous inbred lines. Hybrid vigor, the increase in such characteristics as size, growth rate, fertility, and yield of hybrid organism over those of it parents. Plant and animal breeders explain hetrosis mating two different pure bredlines that have extain desirable traits. The first generation offspring generally show in greater measure, the desired characteristic of both parents. This vigor may decrease, however, if the hybrids are mated together, so the parental lines must be maintained and crossed for each new crop or group desired.